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Tungsten inert gas (TIG) welding is an incredibly accurate and refined form of welding, using an inert gas shield to help achieve perfection. It is used in a variety of different metals where high quality welds and a good surface finish are important. Choosing the right gas shield is important, so can Nitrogen gas be used for TIG welding?
Can you use Nitrogen for TIG welding? Nitrogen can be used in TIG welding but should be combined with Argon gas and only be used for a particular application.
There are a few reasons that this is the case. In this article we are going to go over why Nitrogen needs to be combines with Argon, but we will also explore the basic information that you might need to know when looking shielding gas in TIG welding in general.
Shielding Gas and TIG Welding
Shielding gas prevents the weld pool from being exposed to the atmosphere. The reaction of elements like Oxygen, Hydrogen, and Nitrogen with the weld pool can cause oxidation, weld contamination, and excessive spattering. Typically, the inert gas Argon is used solely as a shielding gas for TIG welding.
Pure Argon works well for most metals, however, a mixture of Argon and Nitrogen has been shown to be effective on a specific type of stainless steel called duplex stainless steel.
What Is Duplex Stainless Steel?
First, to understand what duplex stainless steel is, you need to understand stainless steel itself. There are a few forms of stainless steel to go over as well before we can fully explain what duplex is.
Types of Stainless Steel
Stainless steel is commonly used in situations involving extreme heat and harsh pressure. This material commonly contains a multitude of elements like titanium and nickel, which assists in making it tough and relatively resistant to becoming corroded.
Stainless steel has a reputation for not being easily tarnished. In large part, this can be attributed to its commonly held chromium content of around 11 percent.
Austenitic stainless steel, which contains 16 to 26 percent chromium and up to 35 percent nickel, is steel that was considered to have the highest corrosion resistance. Ferritic stainless steel contains around 10 to 27 percent chromium and is nickel-free, giving it less anticorrosion applications.
When both of these types are combined,
however, they become duplex stainless steel. It contains 21 to 27 percent
chromium, around one to 8 percent nickel, .05 to three percent copper and .05
to 5 percent molybdenum. Duplex stainless steel is stronger and more resistant
to corrosion than austenitic and ferritic steels combined.
While authentic and ferritic stainless steels do well on their own, they had their problems respectively. Duplex stainless steel became popular because it became a way to fill all of those gaps.
It was discovered that when combining these two materials to create duplex stainless steel, it resulted in a material that was higher in strength, lower in weight and had good weldability and increased resistance to corrosion.
Its “duplex” structure gives duplex stainless steel these valuable and sought-after properties:
- Strength – Duplex stainless steel is about twice as strong as regular austenitic or ferritic stainless steels.
- Toughness and Ductility – Duplex stainless steels have significantly better toughness and ductility than ferritic steel.
- Corrosion Resistance – The chromium, molybdenum and nitrogen content make it very corrosion resistant.
- Stress Corrosion Cracking Resistance – Duplex stainless steel shows excellent stress corrosion cracking (SCC) resistance, thanks to its ferritic properties.
- Cost – Duplex stainless steel has lower nickel and molybdenum content, which makes it more economical in cost. Also, due to its increased yield strength, it may be possible to reduce its section thickness for some applications, reducing its price even more.
How Is Duplex Stainless Steel Used?
Duplex stainless steels are gaining greater interest due to their increasing amounts of application fields.
They can be used in highly corrosive environments such as offshore and petrochemical plants, where the integrity of the welded material is essential. Molybdenum-containing alloys, especially duplex stainless steels, last longer than almost any other materials.
Some other applications where duplex stainless steels are being used include:
- Chemical Tankers – These specialized ships carry liquid cargo that may be environmentally hazardous, flammable or highly reactive. Duplex stainless steel tanks are used to transport these liquids.
- Bridges and Buildings – Bridge designers are benefitting from the strength and low-cost of duplex stainless steel. This material is used to build skyscrapers, making them beautiful, sustainable, resilient and durable.
- Dental Instruments – These need to be tough enough not to break during procedures, as well as hygienic, corrosion-resistant, and easily sterilized. Duplex stainless steel works perfectly for these applications.
Why is Nitrogen Used to TIG Weld Duplex Stainless Steel?
Because duplex stainless steel is mainly used in highly corrosive environments where the integrity of the weld material is essential, it is necessary to ensure that the material isn’t compromised during TIG welding.
The integrity of the chemical structure of duplex stainless steel needs to remain intact after weldment in order for it to do its job properly.
Originally, when duplex stainless steel was welded, there was concern over the distribution of ferrite and austenite that formed in the material after it was cooled.
Since the ratio of these two elements profoundly influences corrosion and mechanical properties, it is of the utmost importance to balance them properly when welding. This way, duplex stainless steel will maintain its excellent anti-corrosive properties.
The Effect of Nitrogen Gas
Introducing Nitrogen into the shielding gas has shown to help balance austenite and ferrite. Nitrogen increases corrosion resistance and is a strong austenite stabilizer.
It has also been found to accelerate the partial transformation from ferrite to austenite during cooling after welding.
These factors result in a significant improvement in the integrity of the welded material and its corrosion resistance, in particular.
Also, when compared to components that were welded with an only a pure Argon gas shield, the strength values of materials welded with an addition of Nitrogen were increased.
How is Nitrogen Used to TIG Weld Duplex Stainless Steel?
A mixture of Argon and Nitrogen is specifically designed for welding duplex stainless steel. Nitrogen, in particular, is a big part of its corrosion performance, especially when it comes to pitting corrosion. It can also create a more stable arc.
Since they are used mainly for their good corrosion resistance, a shielding gas that can improve this property will help maintain the design-life of a component, as well as reduce scrapping rates and additional costs.
The addition of just one to two percent of Nitrogen gas into Argon shielding gas will provide the optimal balance and keep duplex stainless steel’s integrity sound.
Nitrogen is an important alloying element when welding duplex stainless steel. Adding it to the Argon shielding gas will also compensate for any loss of nitrogen from the weld pool.
What if Too Much Nitrogen is Used to TIG Weld Duplex Stainless Steel?
Working with Nitrogen is very tricky, however. It can be mixed with Argon to increase the strength and anti-corrosion properties of duplex stainless steel. However, if too much Nitrogen is added to the mixture, it can cause problems.
High Nitrogen content in shielding gas – which is considered anything over three percent – can cause a breakdown of the tungsten. The Nitrogen will increase the erosion of the tungsten electrode and cause tungsten inclusions.
Too much Nitrogen in the gas can also result in a layer of austenite on the weld surface; this can cause a reduction in weld strength as well as less resistance to stress corrosion cracking.
How Do You Properly TIG Weld Duplex Stainless Steel?
Alloy metals such as duplex stainless steel have a lower thermal conductivity than other steel, and their weld puddle will have a lower viscosity.
Because the weld pool is not as fluid, you will need to make adjustments when welding. An important tip when TIG welding duplex stainless steel is to slow down your process. Traveling too quickly can lead to issues.
It is recommended that you don’t try to compensate for a sluggish puddle by increasing your weld current. Using more current will vaporize or burn out the alloying elements, leading to corrosion and reduced life.
Use a low to moderate heat input, visually inspecting all welds for indications of excess heat input (such as color change).
Having a good understanding of what shielding gases work best with each welding process is the best way to always get the best quality welds. This article has hopefully gotten you closer to that understanding by laying out some of the basics of how shielding gas interacts with TIG welding.
There are always going to be more things to know when it comes to welding, however, so learning even more about the use of shielding gas is only going to make you a better welder and your welds a better quality at the end of the day. Sometimes, that knowledge is going to come from trial and error. Just keep welding and eventually, you’ll be able to answer these questions yourself!